Influence of Lignin Addition on Coke Strength

In the steel industry, promoting the use of low-grade coal is important because of the depletion of coking coal. To compensate for the lack of caking property of low-quality coals, binders must be added to produce coke from coal blends. Lignin is an aromatic polymer and one of the main components, accounting for 30% of wood. The chemical structure and characteristics of lignin differ depending on the raw woody biomass and the extraction method used. In this study, to find biomass-derived binders, alkali lignin, sulfuric acid lignin, kraft lignin, organic solvent lignin, phenolated lignin (lignophenol), and torrefied lignophenol were blended at a ratio of 3 mass% with coal powder (coking coal: thermal coal = 1:1). The blended powder was carbonized to form coke, and its bulk density and strength were measured. With the addition of lignin, the bulk density of coke was almost the same or decreased, and the strength decreased significantly, regardless of the lignin type. Lignophenol was expected to be a potential binder because it was observed to be thermoplastic at the melting temperature of coking coal by an in situ camera; however, it largely reduced the fluidity of the coal and the coke strength. The lignin samples did not behave like asphalt pitch, and it was concluded that it was difficult to use lignin as a binder for blended coal.